1. Field of the Invention
The present invention relates to a protection apparatus for safety operation of an automated electric motor-driven sewing machine such as industrial sewing machine.
2. Description of the Prior Art
For the electric motor-driven sewing machine particularly for use on the industrial or commercial base, there have been hitherto known a method of driving the sewing machine through a clutch mechanism including an electromagnetic clutch and brake (e.g. refer to U.S. Pat. No. 3,761,790) as well as a method of directly transmitting power to the sewing machine from a DC motor. However, those methods are basically similar with respect to controlling the sewing machine.
In the modern sophisticated industrial sewing machines, many complicated control procedures are adopted. For a better understanding of the invention, a general description will be made of a control apparatus for a sewing machine which is driven through an electromagnetic clutch and provided with an automatic thread clipping or trimming mechanism as well as an automatic stopping mechanism for stopping the operation of the sewing machine at an upper (top) and a lower (bottom) position of the sewing needle.
In general, command signals for controlling the operation of the sewing machine are produced by a foot pedal. More specifically, when the foot pedal is pressed down in the forward direction, a leading command switch (LSW) is turned on, while a trailing command switch (TSW) is turned on upon treading of the foot pedal in the backward direction. In the neutral position of the foot pedal, both of the leading and trailing command switches are in the off-state. Additionally, an up position detector (UD) for detecting the upper position of the sewing needle is provided in combination with a down position detector (DD) for detecting the lower position of the sewing needle for producing the respective signals. A tachometer generator (TG) is also provided to produce a signal representative of the actual operation speed of the sewing machine, which signal is then supplied to a speed controller circuit to be compared with a reference command signal, whereby the sewing machine is operated at a preselected constant speed under the control of the speed controller circuit.
The output signal from the speed controller circuit includes a first control signal applied to a coil of the clutch mechanism and a second control signal applied to a coil of the electromagnetic brake. Both of these control signals are made valid by a speed control enabling signal produced by a sequence control unit. In other words, only when the speed control enabling signal is present, the speed control is validated, thereby to drive the sewing machine at the selected constant speed.
The sequence control unit operates in general in the following manner:
In the first place, when the leading command switch is turned on by treading down upon the foot pedal forwardly, the speed control enabling signal is produced, whereby the clutch coil and the brake coil are energized in dependence on the output signals from the speed controller circuit to drive the sewing machine at a constant speed. When the foot pedal is actuated to the neutral position, the leading command switch is turned off, resulting in disappearance of the speed control signal. However, if the down position detector produces no lower needle position signal at that time, the speed control enabling signal is produced to rotate the sewing machine until the needle reaches the lower position, whereupon the speed control enabling signal is invalidated by the output signal from the down position detector. At the same time, the brake coil is energized for a predetermined time duration, thereby to stop the sewing needle at the lower position. When the trailing command switch is turned on by pressing down the foot pedal backwardly, the automatic thread trimmer coil is energized to actuate the automatic thread clipping or trimming mechanism. At that time point, no signal is produced from the up position detector. Thus, the speed control enabling signal can be generated to drive the sewing machine.
Upon detection of the upper needle position through the associated up position detector, the automatic thread trimmer coil is deenergized. Simultaneously, the brake coil and the wiper coil for the wiper mechanism to remove or clear the trimmed thread are energized for a predetermined time, thereby to stop the sewing machine at the upper needle position to allow the trimmed thread to be wiped away.
In the electric motor-driven sewing machine, the operations of which are controlled in the manner described above, adequate measures must be taken for the safety and security of the machine operation in view of the fact that the needle which may possibly injure the user is handled in addition to the fact that one sewing machine may be manipulated by a plurality of non-specified workers.
It is to be mentioned in conjunction with the foregoing that both of the up and down needle position detectors UD and DD are mounted on the main body of the sewing machine, while the leading and trailing command switches are mounted at the positions capable of being interlocked with the foot pedal. Thus, the connections of these components to the other circuit and in particular to the speed controller circuit have to be realized by using lead wires. When short-circuit or breakage of these leadwires should occur, the output signals from both of the up and down position detectors or the output signals from both of the leading and the trailing switches will be produced simultaneously, as the result of which the automatic thread trimmer mechanism and the thread wiper mechanism may be erroneously actuated notwithstanding of the fact that the needle is located at the lower position, thereby involving breakage of the needle, injury to the sewing machine and the burning of the drive motor. These undesirable phenomena may be caused by noises.